There are many instances in which it is highly desirable to apply electrostatic charges to surfaces to be sensitized by such charges or to remove electrostatic charges from material surfaces. Electrode arrangements for applying or removing such charges are well known and have been extensively used. In those instances where the entirety of the surface is to be sensitized or is to have the electrostatic charge removed therefrom, such prior electrode arrangements have been quite satisfactory.
However, in many cases, especially in situations where sheet material is moved continuously, it is desirable to remove such electrostatic charges from only individual sections or areas of the moving sheet surface. Also, in many other cases, especially in intaglio or gravure printing applications in which electrostatic charges are used in aiding in the printing process, it is highly desirable to provide the impression cylinder with an electrostatic charge only in those areas in which color is to be transferred between the printing cylinder and the printing substrate. It is also highly desirable that the field strength of the electrostatic charge applied to the impression cylinder be distributed as uniformly as possible for optimal transfer of printing ink from the printing cylinder to the substrate, particularly to obtain the desired inking for semi-tone and ink mixtures.
While known electrode arrangements for sensitizing surfaces by electrostatic charges, e.g. the impression cylinder of a gravure printing press, have included means for controlling the supply voltage with respect to the electrostatic charge being applied to the surface for controlling the field strength and such electrode arrangements do operate satisfactorily in this respect, many of these known electrode arrangements also apply an electrostatic charge to the surface outside of the specific area or areas desired to be sensitized. Such attributes of these known electrode arrangements are undesirable if continuous satisfactory operation is to be obtained. With specific reference to intaglio printing, there are two reasons for this undesirability, namely that the electrostatic charge field tends to progressively widen from the point of application toward the ink transfer zone while the load intensity weakens, and that the impression cylinder surface outside of the area which contacts the substrate experiences a relatively fast buildup of undesirable ink applications. Of course, such undesirable contamination of the impression cylinder surface must be frequently removed through cleaning resulting in interruptions to the printing operation, but also the field or load intensity weakening process increases successively and thus printing quality is impaired.
There have been prior attempts to solve these problems through use of push-on or plug-in type insulating covers which may be used to cover those portions of the electrode arrangement corresponding or opposite the portions of the surface which are not to be charged. However, since the covered electrode points continue to be activated even though covered, the covers themselves become sensitized with an electrostatic charge to an extent that dust and ink may quickly settle on the covers which results in the same undesirable contamination as with the impression cylinder surface. Another prior attempt has suggested an electrode arrangement in which certain electrodes may be individually and selectively energized or de-energized through the use of manually operable contact points. However, such electrodes must be energized or de-energized in a particular sequence or pattern and the activation or deactivation thereof can only be done on an individual and predetermined basis.